All-fiber module for femtosecond pulse compression and supercontinuum generation

ABSTRACT

An all-fiber optical pulse compression arrangement comprises a concatenated arrangement of a section of input fiber (e.g., a single mode fiber), a graded-index (GRIN) fiber lens and a section of pulse-compressing fiber (e.g., LMA fiber). The GRIN fiber lens is used to provide mode matching between the input fiber (supporting the propagation of chirped optical pulses) and the pulse-compressing fiber, with efficient pulse compression occurring along the length of the LMA fiber. The dispersion and length of the LMA fiber section are selected to provide the desired degree of pulse compression; for example, capable of reconstituting a femtosecond pulse as is used in supercontinuum generation systems.

TECHNICAL FIELD

The present invention relates to an arrangement for providing pulsecompression (and subsequent continuum generation) at the output of afemtosecond laser source and, more particularly, to an all-fibercompression arrangement utilizing a graded-index fiber lens disposedbetween a first fiber section (for propagating incoming chirped pulses)and a second fiber section (for compressing the chirped pulses).

BACKGROUND OF THE INVENTION

Fiber lasers with high pulse energy, good beam quality and excellentoptical characteristics have applications in many fields and industries,such as analytical spectroscopy (e.g., fluorescence, absorption),illumination, remote sensing and environmental spectroscopy (e.g., windspeed, biohazards, eco-system mapping, etc.), ranging and targeting(e.g., collision avoidance, military applications, etc.) and scientificinstrumentation. Fiber lasers with exceptionally short pulse widths, forexample, femtosecond fiber lasers, have special applications in theseand other fields.

There has been great progress in developing short pulse fiber lasers.One approach is to use nonlinearity during amplification in thewavebreaking-free regime of normal dispersion amplifiers to generate achirped pulse. Pulse compression can then be performed in a coupledsection of single mode fiber. U.S. Pat. No. 6,990,270 issued on Jan. 24,2006 to J. Nicholson and assigned to the assignee of this application isexemplary of this type of arrangement. However, one of the difficultiesassociated with femtosecond pulses in fibers is compressing the highenergy pulses. Nonlinearities in the fiber create distortions in thespectrum, causing the pulse to lose energy to undesirable pedestals or,worse, break up into multiple satellite pulses.

Often, “stretched” pulse amplification is implemented, where theultrashort pulse is first stretched in the time domain by many orders ofmagnitude, temporally broadening the pulse and decreasing the peakpower. The stretched pulse is then amplified, eliminating or reducingthe nonlinear interactions present when attempting to amplifyfemtosecond pulses. However, whether using a similariton-type amplifier,or stretched pulse amplification, the chirped, amplified output pulsemust ultimately be re-compressed, where the high pulse energiesassociated with amplification means that the recompression stage isusually done using bulk optics.

A fiber that is capable of propagating and compressing high energyfemtosecond pulses would then be desirable for two reasons. First, ifthe fiber can be designed with an appropriate dispersion, it could serveas a compression stage for stretched, high energy pulses. Second, if thecompression function can be implemented in a fiber, it can also serve asa delivery fiber for ultrashort pulses for a wide variety ofapplications, as mentioned above.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention which relates to an arrangement for providing pulsecompression at the output of a chirped femtosecond pulse source and,more particularly, to an all-fiber compression arrangement utilizing agraded-index fiber lens disposed between a first fiber section (forpropagating incoming chirped pulses) and a second fiber section (forcompressing the chirped pulses). The fibers are selected such that theeffective area of the pulse-compressing fiber (denoted A_(eff-2)) isgreater than the effective area of the input fiber (denoted A_(eff-1))at the system wavelength. In one embodiment, the pulse-compressing fibermay comprise a section of a large-mode area (LMA) fiber, with the inputfiber comprising a section of standard single mode fiber.

In accordance with the present invention, a fiber-based graded-indexlens is disposed between a fiber exiting a pulse stretching unit and asection of pulse-compressing fiber. The graded-index fiber lens (i.e.,GRIN lens) comprises a section of fiber with a radially-dependentprofile index profile (for example, a parabolic index profile) and alength appropriate to form a quarter-pitch lens (or any odd multiplethereof). Often the GRIN fiber lens is formed from a short section ofmulti-mode fiber. The GRIN fiber lens thus provides matching of themodefield diameters between the input fiber and the pulse-compressingfiber in a preferred “all-fiber” arrangement. The dispersion (positive)and length of the pulse-compressing fiber are selected to provide thedesired degree of pulse compression; for example, capable ofreconstituting a femtosecond pulse train as is used in supercontinuumgeneration systems.

The utilization of an all-fiber pulse compression arrangement eliminatesthe need for bulk optic components. Therefore, the transmission lossesassociated with the in-line arrangement are considerably less than thosefound in prior art pulse compression arrangements using bulk components.

In a further embodiment of the present invention, the pulse-compressingfiber may be coupled to a further section of transmission fiber (using,preferably, a second GRIN fiber lens to provide matching of modefielddiameters), to deliver the amplified femtosecond optical pulse train toits intended application. In one particular embodiment, a section ofhighly-nonlinear fiber (HNLF) may be coupled to the pulse-compressingfiber and create a supercontinuum from the generated pulse train offemtosecond pulses.

Other and further embodiments of the present invention will becomeapparent during the course of the following discussion and by referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, where like numerals represent like partsin several views:

FIG. 1 illustrates an all-fiber pulse compression arrangement formed inaccordance with the present invention;

FIG. 2 is a diagram illustrating the basic principles of a graded-indexfiber lens;

FIG. 3 illustrates an exemplary fiber lens arrangement as used in thepulse compression arrangement of the present invention;

FIG. 4 contains graphs of pulse shape at the output of a pulsecompressor, with the graph of FIG. 4( a) associated with a prior artpulse compression arrangement using single mode fiber and the graph ofFIG. 4( b) associated with the inventive pulse compression arrangementusing a section of large-mode area fiber;

FIG. 5 contains plots of the measured pulse correlation width as afunction of the length of the fiber section used to perform the pulsecompression, with the plot of FIG. 5( a) associated with various lengthsof single mode fiber (prior art) and the plot of FIG. 5( b) associatedwith various lengths of pulse-compressing fiber of the presentinvention;

FIG. 6 illustrates an alternative embodiment of the present invention,including an output GRIN fiber lens coupled to the output of thepulse-compressing fiber, the arrangement of FIG. 6( a) using a singlesection of fiber and the arrangement of FIG. 6( b) using a concatenatedpair of fibers spliced together;

FIG. 7 contains plots of the measured spectra associated with thearrangements of FIGS. 6( a) and 6(b);

FIG. 8 illustrates an exemplary supercontinuum generating arrangementformed in accordance with the present invention;

FIG. 9 contains plots of the continuum generated by the arrangement ofFIG. 8, as compared to prior art-generated continuum;

FIG. 10 shows an exemplary asymmetric interferometer arrangement thatmay be used to measure the cross-coherence associated with the continuumgenerated in the arrangement of the present invention as illustrated inFIG. 8; and

FIG. 11 contains plots of the cross-coherence measured in thearrangement of FIG. 10, one plot associated with the prior art SMFcompression arrangement and the other associated with the all-fiberpulse compression arrangement of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary all-fiber femtosecond pulse compressor10 formed in accordance with the present invention. As shown, compressor10 comprises a graded index (GRIN) fiber lens 12 disposed between aninput fiber 14 and a section of pulse-compressing fiber 16. Fibers 14and 16 are selected such that the effective area of fiber 14 (denoted asA_(eff-1)) is less than the effective area of fiber 16 (denoted asA_(eff-2)) at the operating wavelength. A source of ‘stretched’femtosecond pulses 18 (alternatively referred to in the art as ‘chirped’pulses), formed in a manner well-known in the prior art, is shown inconjunction with compressor 10 in FIG. 1. Source 18 is used to create atrain of chirped pulses P which is thereafter coupled into input fiber14. As discussed above, ultrashort pulses (on the order of 100femtoseconds, for example) may first be stretched in the time domain bymany orders of magnitude, temporally broadening the pulse and decreasingthe peak power to essentially eliminate (or at least reduce) nonlinearinteractions during amplification.

Optical pulse compressor 10 of the present invention may be utilizedafter amplification to re-compress the pulses into their originaltemporal form. In particular, GRIN fiber lens 12 is used to provide ahigh quality coupling from the output of input fiber 14 intopulse-compressing fiber 16 by performing mode-matching between the twofibers. Pulse-compressing fiber 16 is selected to exhibit a known(positive) dispersion characteristic D at the operating wavelength, andis formed to exhibit a predetermined length L that has been found toprovide the desired amount of pulse compression (e.g., to formfemtosecond pulses). Thus, the output from pulse-compressing fiber 16(i.e., the output from optical pulse compressor 10) is a train ofamplified femtosecond pulses. It is to be noted that the length L offiber 16 must be selected so as to compensate for the spectral phase ofthe launched pulse. This length consideration, which is based on thedispersive properties of the selected pulse-compressing fiber, isgenerally not a consideration in typical high power pulse amplifiers,but is considered to be an essential aspect of the arrangement of thepresent invention, and necessary to provide pulse compression with thedesired linear properties.

Advantageously, the inclusion of GRIN fiber lens 12 between fibers 14and 16 eliminates the need for a bulk optic device, as was used in theprior art, to couple the pulses into the pulse-compressing fiber. Bulkoptics are known to introduce loss, scattering, reflections and thelike, all reducing the quality of the signal launched into thepulse-compressing fiber. In contrast, the use of a fiber-based componentsuch as GRIN fiber lens 12 significantly reduces the various couplinglosses and allows for the launched pulses to achieve low levels ofmulti-path interference (MPI) and low nonlinearity compression in LMAfiber 16.

Additionally, the use of an in-line fiber coupling arrangementeliminates the alignment problems associated with the use of bulkoptics, since permanent alignment between the GRIN fiber lens and thepulse-compressing fiber is automatically achieved when the two are fusedtogether (see, for example, U.S. Pat. No. 4,701,011 for a discussion ofself-alignment between transmission fibers and GRIN fiber lenses).Another advantage of using an in-line coupling arrangement is theintrinsic isolation of the optical signal path from dust or otherextrinsic contaminants, as may be found in bulk optic solutions.

The ability to use a fiber-based GRIN lens to provide mode couplingbetween input fiber 14 and pulse-compressing fiber 16 is, therefore, acritical component of the present invention. FIG. 2 contains a basicdiagram illustrating the principles associated with the implementationof a graded-index fiber lens. In particular, FIG. 2 illustrates aGaussian beam 100 exiting a single mode fiber 110 and thereafter passingthrough a parabolic-index medium 120, which is preferably a section ofmultimode fiber. The waist position and beam size associated withGaussian beam 100 may be found from equations well-known in the art. Inorder to provide mode-matching into the pulse-compressing fiber, asdesired in the arrangement of the present invention, beam 100 shouldachieve maximum expansion at Z=0, the exit of parabolic-index medium120. Thus, medium 120 should comprise a length L equal to π/2 g, where gis the focusing parameter of medium 120. These parameters result in theformation of a device commonly referred to in the art as a “quarterpitch” lens (or, in general, any odd multiple thereof). FIG. 3illustrates an exemplary fiber-lens arrangement providing this maximumexpansion suitable for mode matching, showing a section of multimodefiber 210, of length L, fused to endface 220 of a section of single modefiber 230. As shown, the length L is associated with the Z=0 point ofmaximum expansion.

It is to be understood that this discussion of a “parabolic index”embodiment is exemplary only, and for the purposes of explanation, notlimitation, since a graded-index fiber lens formed in accordance withthe present invention may utilize any refractive index gradient that iscapable of achieving mode-matching between the single mode fiber and thelarge mode area fiber.

FIG. 4 contains graphs illustrating the improvement in compressed pulseshape that may be achieved when using the GRIN lensfiber/pulse-compressing fiber arrangement of the present invention, ascompared to the prior art use of only a section of single mode fiber.The plot of FIG. 4( a) shows the autocorrelation function associatedwith a compressed pulse train created using only a section of singlemode fiber (prior art). The original ‘stretched’ pulses were generatedby amplifying the output of a passively mode-locked fiber laser. Thecombination of normal dispersion and self-phase modulation in thesingle-mode, erbium-doped fiber amplifier creates pulses with a strongnegative chirp.

To create the results shown in FIG. 4( a), the output section of singlemode fiber (performing the compression function) was continually reducedin length until the shortest pulse output was found. In this case, for a4 nJ pulse, compression in single mode fiber is shown to includesignificant sidelobes in the autocorrelation function; expected as aresult of the inherent nonlinearity of the single mode fiber.

In contrast, the plot of FIG. 4( b) shows the autocorrelation functionof output pulses from an all-fiber pulse compressor formed in accordancewith the present invention. For this particular embodiment, a section oflarge-mode area (LMA) fiber was used as the pulse-compressing fiber. TheLMA fiber was selected to exhibit an effective area A_(eff-2) of 986 μm²at the operating wavelength of 1550 nm, with a dispersion of +21.08ps/nm-km and a dispersion slope of 0.063 ps/nm²-km. The positivedispersion value at the operating wavelength compensates for the normaldispersion of the amplifier fiber, as well as the additional nonlinearphase due to self-phase modulation during amplification in single modefiber. As shown, the generated autocorrelation function exhibitsessentially no sidelobes, indicative of the elimination ofnonlinearities in the all-fiber pulse compression arrangement of thepresent invention.

As mentioned above, the dispersion and length of the section of thefiber used to provide pulse compression are critical factors indetermining the amount of pulse compression that may be achieved. Theplots of FIG. 5 illustrate the pulse width correlation as a function ofthe length of the compression fiber. FIG. 5( a) illustrates thiscorrelation as a function of fiber length for a prior art pulsecompressor using single mode fiber, and FIG. 5( b) shows the samecorrelation as a function of length for an exemplary LMA fiber used thepulse-compressing fiber in an exemplary embodiment of the presentinvention. Referring to FIG. 5( b), the correlation is seen to achieve aminimal value of about 100 fs for a length L of about 1.7 m, with anaccuracy of approximately 5 cm required to achieve the minimum pulsewidth. A similar accuracy and fiber length is also required for pulsecompression in prior art arrangements using single mode fiber; however,as discussed above, nonlinearity in the single mode fiber causes thegeneration of significant wings in the autocorrelation. Thus, thearrangement of the present invention is seen to provide a more faithfulreproduction of the original femtosecond pulses than could be achievedby using only single mode fiber. One of ordinary skill in the art willrecognize that the use of ultrashort pulses, by nature of the FourierTransform relation between time and frequency, requires that the deviceoperate over a broad bandwidth. For example, FIG. 5 a shows a pulsecentered at 1550 nm with a width on the order of 1 ps, which, iftransform limited, would require a spectrum with a bandwidth (full-widthat half-maximum) of approximately 2.5 nm. Chirped pulses, or transformlimited pulses with shorter widths would require even more bandwidth.

If the compressed output pulses from the arrangement of the presentinvention are to be thereafter propagated along a transmission fiber(which is likely for medical applications, sensing applications, etc.),a preferred embodiment in accordance with the present invention includesthe use of a second GRIN lens between the output of thepulse-compressing fiber and a transmission fiber to provide efficientmode matching between the compression arrangement and the transmissionarrangement. FIG. 6 illustrates this embodiment of the presentinvention, providing a fully all-fiber pulse compression andtransmission system.

FIG. 6( a) shows a first arrangement where a second GRIN lens 18 isdisposed between the endface of pulse-compressing fiber 16 and a singlemode output transmission fiber 20. Preferably, second GRIN lens 18 isfused between fibers 16 and 20, since fusion provides automaticalignment between the core regions of the various fibers. As with theproperties of GRIN lens 12 discussed above, second GRIN lens 18 isformed to have a length L′ suitable for providing mode-matching betweenpulse-compressing fiber 16 and SMF 20. The alternative arrangement asshown in FIG. 6( b) includes two separate sections of pulse-compressingfiber, shown as 16-1 and 16-2, with a splice formed therebetween tocouple the fibers together.

Multi-path interference (MPI) is a measure of the mount of light inundesired, higher-order modes and is defined as follows:MPI=10*log(P _(HOM) /P _(F)),where P_(HOM) is the total optical power propagating in undesiredhigher-order modes of the compressed fiber, and P_(F) is the opticalpower in the fundamental mode. FIG. 7 contains plots of the measuredspectra for the arrangements of FIGS. 6( a) and (b). The measured MPI ofthe arrangement of FIG. 6( a) was −31 dB, and the MPI for the splicedarrangement of FIG. 6( b) was −26 dB, showing the majority of the poweris contained in the desired fundamental mode. Achieving low MPI isimportant for a number of reasons. First, large amounts of energy inundesired higher-order-modes can lead to signal power fading as thephase between coherent modes drifts. Additionally, MPI can potentiallylead to increased noise of the compressed pulses are to be used infurther nonlinear processes such as supercontinuum generation.

Given this ability to couple the femtosecond pulses into an outputsingle mode fiber via a second GRIN lens, it is further possible tothereafter couple a section of highly nonlinear fiber (HNLF) to thesecond GRIN lens (or alternatively, the section of output signal modefiber) to form an “all-fiber” arrangement for supercontinuum generation.As described in various prior art references, including U.S. Pat. No.6,775,447 (Nicholson et al., assigned to the assignee of thisapplication), the propagation of extremely short pulses (such asfemtosecond pulses) through one or more sections of HNLF will generate acontinuum of extremely broad bandwidth, useful in frequency metrologyand other applications (such as DWDM).

FIG. 8 illustrates an exemplary supercontinuum generation system asformed in accordance with the present invention. In this particulararrangement, a section of highly-nonlinear fiber (HNLF) 22 is spliced toSMF 20, using the arrangement of FIG. 6. It is to be understood that inthe alternative HNLF 22 may be directed fused to the endface of secondGRIN fiber lens 18. Moreover, HNLF 22 may comprise a plurality ofconcatenated sections of fiber, each having different dispersioncharacteristics to provide a broad continuum. FIG. 9 contains plots ofthe generated continuum associated with the inventive arrangement ofFIG. 8, as compared to a prior art continuum generation arrangementwhere the pulses are first compressed in a section of single mode fiberand then launched into a section of HNLF. From the spectra as shown,both pulse compression techniques produce very broad, smooth spectra inshort lengths of HNLF. Other factors, such as the presence of noisealong the spectra, cannot be ascertained from this measurement.

Thus, in order to measure noise along the generated continuum, across-coherence measurement can be performed by interfering twoindependent continua and measuring the fringe contrast as a function ofwavelength. FIG. 10 illustrates an exemplary asymmetric interferometer50 that may be used to measure the cross-coherence associated with eachof the generated continuum. Interferometer 50 receives as an input thecontinuum produced by a section of HNLF, such as HNLF 22 shown in FIG.8. A beam splitter 52 is used to create two independent continua, afirst continuum propagating along a first path 54 and a second continuumpropagating along a second path 56. Interferometer 50 is ‘asymmetric’ byforming second path 56 to be unbalanced with respect to first path 54 bya length equal to the distance between pulses in the incoming pulsetrain, as shown. The pulses then overlap within a beam combiner 58 atthe output and are passed through a polarizer 60 and coupled into asection of single mode fiber 62. The use of a polarizer at the outputensures polarization overlap, and the single mode fiber ensures modaloverlap. A variable neutral density filter 64 is included along firstsignal path 52 and a quarter-wave plate 66 along second signal path 54allow for power equalization between the two paths. An optical spectrumanalyzer 68 is thereafter used to measure the fringe contrast atdifferent wavelengths.

FIG. 11 shows the measured cross-coherence of the continua as plotted inFIG. 9. For perfectly coherent spectra, the fringe contrast will beequal to “1” (i.e., unity). Any degradation in coherence (as would beassociated with the presence of noise in the generated supercontinuum)results in lowering the fringe contrast. FIG. 11 has plotted the fringecontrast on a dB scale to better show the difference in pulsecompression between the prior art SMF pulse compression and the use ofthe GRIN lens/LMA combination of the present invention.

As shown in FIG. 9, continua from both the prior art and inventivecompressors show very high coherence, as expected. However, by review ofthe data of FIG. 11, it is clear that the LMA compressor formed inaccordance with the present invention shows a fringe contrast of almost20 dB (0.99 fringe visibility on a linear scale) over a substantialportion of the wavelength range. In comparison, the pulses compressedusing the prior art SMF arrangement show an almost 10 dB degradation infringe contrast, and at one micron the fringe contrast on a linear scaleis reduced from 0.99 to 0.92.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alternativesand modifications will no doubt become apparent to those skilled in theart after reading the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alternatives andmodifications as fall within the spirit and scope of the presentinvention.

1. An optical pulse compressor comprising: a first section of fiberexhibiting a first effective area A_(eff-1) at an operating wavelength,said first section of fiber supporting the propagation of a chirpedoptical pulse; a graded-index fiber lens coupled to the output of thefirst section of fiber, wherein the graded-index fiber lens exhibits anapproximately parabolic-shaped graded index profile; and a secondsection of fiber exhibiting a second effective area A_(eff-2) at theoperating wavelength, where A_(eff-2)>A_(eff-1), and further exhibitinga dispersion D and a length L selected to compress the chirped opticalpulse, said second section of fiber coupled to the output of thegraded-index fiber lens, wherein the graded-index fiber lens providesmode matching between said first and second sections of fiber over abandwidth of greater than 2 nanometers, the bandwidth is defined atfull-width half-maximum.
 2. An optical pulse compressor as defined inclaim 1 wherein the graded-index fiber lens comprises a quarter-pitchfiber lens or any odd multiple thereof.
 3. An optical pulse compressoras defined in claim 1 wherein the first section of fiber comprises asection of single mode fiber.
 4. An optical pulse compressor as definedin claim 1 wherein the second section of fiber comprises a section oflarge-mode area (LMA) fiber.
 5. An optical pulse compressor as definedin claim 1 wherein the dispersion, D, and length, L, of the secondsection of fiber are selected to produce a femtosecond optical pulse. 6.An optical pulse compressor as defined in claim 1 wherein the secondsection of fiber exhibits multi-path interference of less than −15 dB.7. An optical pulse compressor as defined in claim 1 wherein the inputof the graded-index fiber lens is fused to the output of the firstsection of fiber.
 8. An optical pulse compressor as defined in claim 1wherein the output of the graded-index fiber lens is fused to the inputof the second section of fiber.
 9. An optical pulse compressor asdefined in claim 8 wherein the input of the graded-index fiber lens isfused to the output of the first section of fiber.